What causes the formation of plasma in stellar structures?
Plasma is formed by high temperatures or strong magnetic fields.
Plasma is often referred to as the fourth state of matter. A plasma consists of positive and negatively charged particles which interact with each other but are not bound.
A plasma is formed when temperatures are high enough to strip electrons from atoms. Strong magnetic fields can also do this. The electrons are no longer bound to the now positively charged atoms. It is like a sea of positively charged atoms and unbound negatively charged electrons.
Temperatures inside stars are more than sufficient to create plasma.